Computer runs slow when playing high resolution games

When i try to play Call of duty:World at war and some other games they gave me slow performance during the play, frame rates are too slow. Even with minimum settings frame rates r too slow. Even need for speed and some other games r giving me slow performance at 1024×768 resolution. Some other games like dead space gives me good frame rate but there are some audio problem too. Then only game which i could play properly is Fear. I play games only in XP. In vista these games are not playable.
I tried re-installing drivers for my graphics card, sound and i even installed the sp3 updates for windows
What could b the problem ?
Also my processor is making a lot of noise and the temperature of CPU is also very high. Should i buy a new heat sink and fan?
To upgrade my Graphic card to get better performance?
